There are two well established methods of effecting the counting of sheets moved by the above described apparatus. In one known method a pressure-sensitive counting device is provided in the vacuum supply line between a source of suction and the port faces of the vacuum sheet removing organs. For example, a diaphragm or bellows type device in communication with a vacuum supply line may be arranged to generate count pulses in accordance with pulsations of vacuum level created during the transference of the sheets. It will be appreciated that in this arrangement it will be of no consequence to the counting operation if a suction organ fails to remove a sheet during a counting operation.
The second known method, which is relevant to the present invention, involved theuse of a holding circuit which is maintained by a pressure-sensitive switch disposed in the vacuum supply line. Such a system is disclosed in U.S. Pat. No. 2,912,242 and which includes, inter alia, a means for generating count pulses and for counting the same upon the rotation of the rotary assembly, in conjunction with a pressure-sensitive switch which is arranged to arrest operation of the apparatus and to inhibit counting if the level of vacuum falls below a predetermined value. This arrangement necessitates the provision of means for correctly aligning a suction organ with respect to the stack prior to counting but offers certain advantages over the above-mentioned first method of counting and insures that the machine cannot operate at an inadequate suction level.
Thus the second method effectively serves to prevent the creation of a spurious count pulse by the passage of a suction organ not carrying a sheet and is perfectly satisfactory when used in conjunction with normal grades of paper. However, we have found in practice that when handling banknotes of an unusual nature, for example banknotes made from relatively stiff grade of paper, or badly crumpled, worn or distorted banknotes, that occassions arise when the vacuum level falls below the said predetermined value in spite of the fact that the banknote is correctly transferred by a sheet removing organ. Such an occurence simulates the condition of the passage of a suction organ not carrying a banknote and therefore arrests operation of the apparatus. We have ascertained that the loss of vacuum level is attributable to the delay in the initial sealing of the port face of a suction organ by a banknote, i.e. during the commencement of a banknote feeding cycle. Accordingly, the aim of the present invention is to overcome inhibition of the apparatus during a counting operation while still retaining the advantages of the holding-circuit method.